Storage devices, such as a magnetic medium based disk drive, can store data on circular, concentric tracks on a disk surface. A drive head, such as a read-write head, retrieves and records data on a magnetic layer of a rotating disk as the head flies on a cushion of air over the disk surface. When retrieving data, magnetic field variations are converted into an analog electrical signal, the analog signal is amplified, converted to a digital signal, and interpreted. To guarantee the quality of the information saved on and read back from the disk, the read-write head should be accurately positioned at the center of the track during both writing and reading, and the speed or frequency of read and write should be accurately controlled with respect to the rotating disk. A closed-loop control system can respond to servo information embedded in dedicated portions of a track on the recording surface to accurately position the head and synchronize the timing of drive operations such as read and write operations.
The servo information defines the position of the data tracks and is written with great accuracy in order for a closed-loop control system to operate properly. Typically, servo information is written on a disk surface as a radially extending set of spokes or wedges, e.g., servo wedges. In some implementations, a portion of a servo wedge at a particular track location can include a synchronization field, index mark, and a track number. Head positioning relative to a track center can be determined and corrected, if necessary, by reading and noting the respective amplitudes and timings of the offset bursts.
Traditionally, a machine called a servo writer is used to write the servo information on the disk surface. Commonly, a servo writer uses a large, massive granite base to minimize the effects of vibration. The servo writer can also use precision fixtures to hold the target drive, a precision, laser-interferometer-based actuator arm positioning mechanism to place the arms radially with respect to the axis of rotation of the disks in the drive, and an external clock head to position the servo wedges in time. Present servo writers are typically large and expensive, and as the typical track density increases, the servo writing time also increases, which can create a bottleneck in the disk drive manufacturing process at the servo writer station.
To reduce the time required by the servo writer, techniques such as self-servo writing (SSW) have been developed. Instead of using a dedicated servo writer to slowly write the servo information to each concentric data track on each surface of each disk in a hard drive, a SSW process enables a disk drive itself to write servo patterns to the disk surface based on position marks that are pre-written to the disk surface when the drive is assembled. The position marks are used for the disk drive to locate the positions to write servo pattern, and the marks are written to the disk by a machine which is simpler and cheaper than a dedicated servo writer. Without the aid of the servo writer, a disk drive itself can use information derived from the position marks to determine the radial and circumferential position of the head in order to write conventional servo information to the disk surface.